The UK's most complete GLP-1 guide
Every Question Answered. Every Side Effect Explained.
- How GLP-1 medications work
- Mounjaro vs Wegovy compared
- Injection instructions
- Side effects & solutions
- What to eat on treatment
- FAQs answered
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Should Mounjaro Be Stored In The Fridge?
Yes, Mounjaro should be stored in the fridge before first use. Unopened KwikPen devices must be kept in the refrigerator at 2°C to 8°C to maintain stability and effectiveness. Once you start using a pen, storage rules change — you can keep it at room temperature for up to 30 days. Understanding proper storage is essential for anyone prescribed this tirzepatide injection, whether through NHS specialist services or private prescription. Incorrect storage can damage the medication and reduce its effectiveness.
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Enter your height and weight below. Supports both imperial (stone & lbs) and metric (kg). Results include NHS category, healthy weight range, and ethnicity adjustments.
What Is BMI and Why Does It Matter?
Body Mass Index (BMI) is the most widely used screening tool for assessing whether your weight falls within a healthy range. Used by the NHS, GPs, and weight management services across the UK, BMI provides a quick snapshot of your weight relative to your height. The formula is simple: your weight in kilograms divided by your height in metres squared (kg/m²).
While BMI isn't a perfect measure — it doesn't account for muscle mass, bone density, or fat distribution — it remains the standard first step in identifying potential weight-related health risks including type 2 diabetes, cardiovascular disease, high blood pressure, and certain cancers.
The NHS also recommends adjusted thresholds for people of South Asian, Chinese, or Black African heritage — a BMI of 23 or above is classified as overweight, and 27.5 as obese. Our calculator includes this adjustment as a one-click toggle.
Mounjaro Storage Requirements: Before and After First Use
Before first use (unopened KwikPen):
- Store in the refrigerator at 2°C to 8°C (standard fridge temperature)
- Keep in the original carton to protect from light
- Never freeze — if frozen, the pen is no longer safe to use
- Can remain in the fridge until the expiry date printed on the pen
- Check the temperature of your fridge with a thermometer to ensure it stays within range
After first injection (opened KwikPen):
- Can be stored at room temperature below 30°C
- Can be used for up to 30 days after first use
- Keep away from direct heat and sunlight
- Do not refrigerate after first use — room temperature is preferred
- Dispose of the pen 30 days after first injection, even if medication remains
This differs from Wegovy, which follows similar refrigeration rules but may have slight variations in room temperature tolerance. Always check your specific product leaflet, and ask your prescriber or pharmacist if unsure about your particular medication batch.
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How It Works
Three simple steps to understand your weight and take action.
Enter your height and weight in our free calculator. Get your BMI, NHS category, healthy weight range, and ethnicity-adjusted thresholds instantly.
If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
Use our progress tracker to log your weight weekly, visualise your journey with charts, and stay motivated — all stored privately in your browser.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Why Proper Storage Matters for Mounjaro Effectiveness
Tirzepatide (the active ingredient in Mounjaro) is sensitive to temperature extremes. Storing it correctly preserves its effectiveness — storing it incorrectly can reduce how well the medication works for appetite regulation and weight management.
If your fridge is too warm or too cold, the medication may degrade. If an unopened pen has been left at room temperature for more than a few hours, ask your pharmacist whether it's still safe to use — do not assume it is.
When travelling, keep unopened pens in a fridge-safe cool bag with ice packs, or use a portable medication cooling case designed for injectable medications. After first use, an opened pen is more stable and can travel at room temperature for short periods, making dose administration more flexible during work or social activities.
- Dual GIP/GLP-1 receptor agonist
- Up to 22.5% body weight loss (SURMOUNT-1)
- Weekly injection, dose escalation over 20+ weeks
- Available via private prescription in the UK
- Doses: 2.5mg → 5mg → 7.5mg → 10mg → 12.5mg → 15mg
- GLP-1 receptor agonist
- Up to 15–17% body weight loss (STEP trials)
- Weekly injection, 5-step dose escalation
- NHS & private prescription available
- Doses: 0.25mg → 0.5mg → 1mg → 1.7mg → 2.4mg
Mounjaro vs Wegovy: Head-to-Head
A side-by-side comparison of the two most popular GLP-1 weight loss medications available in the UK.
| Feature | Mounjaro | Wegovy |
|---|---|---|
| Drug class | Dual GIP/GLP-1 agonist | GLP-1 agonist |
| Active ingredient | Tirzepatide | Semaglutide |
| Avg. weight loss | 15–22.5% | 12–17% |
| Key trial | SURMOUNT-1 (2022) | STEP 1 (2021) |
| Max dose | 15 mg weekly | 2.4 mg weekly |
| Escalation period | 20–32 weeks | 16–20 weeks |
| Administration | Weekly injection (pen) | Weekly injection (pen) |
| NHS availability | Private prescription only | NHS & private |
| Approx. private cost | £150–£250/month | £150–£300/month |
| Common side effects | Nausea, diarrhoea, constipation | Nausea, vomiting, diarrhoea |
Data sourced from SURMOUNT-1 and STEP 1 clinical trials. Costs are approximate and vary by provider. Always consult a prescriber.
Am I Eligible for Weight Loss Medication?
In the UK, eligibility for GLP-1 weight loss medications like Mounjaro and Wegovy is based on your BMI and overall health profile.
- BMI of 30 or above (obese)
- Age 18 or over
- No contraindicated conditions
- Committed to lifestyle changes
- BMI of 27 or above
- Plus type 2 diabetes
- Or high blood pressure
- Or obstructive sleep apnoea
Use our BMI calculator to check whether your BMI falls within the eligible range.
How We Built These Tools
Every calculator and guide on myweightcheck is built with accuracy and transparency in mind. Our BMI calculator uses the standard WHO formula with NHS-aligned category thresholds. Our GLP-1 projections are modelled on published clinical trial data from SURMOUNT and STEP studies.
All data stays on your device. We don't require sign-ups, and we don't store your health information on any server. Your progress tracker saves data locally in your browser using localStorage, so only you can see it.
We're based in the UK and our tools are designed specifically for UK users — supporting stone & lbs by default, referencing NHS guidelines, using NICE-approved medication data, and linking to GPhC-registered pharmacies.
Frequently Asked Questions
Common questions about BMI, weight loss medications, and our free UK tools.
Can I store Mounjaro in a regular kitchen fridge?
Yes, absolutely. A standard household refrigerator at 2–8°C is perfect for unopened Mounjaro KwikPens. Keep the pen in its original carton to protect it from light. Check your fridge temperature occasionally to ensure it stays within the correct range.
What happens if Mounjaro freezes?
If your Mounjaro pen freezes, it is no longer safe to use and should be disposed of. Freezing damages the tirzepatide medication. Ensure your fridge is set correctly and avoid placing pens near the freezer compartment where temperatures may drop below 2°C.
Can I store Mounjaro at room temperature before first use?
No. Unopened Mounjaro pens must be refrigerated at 2–8°C before first use. Room temperature storage before opening will damage the medication. Only after the first injection can you store the opened pen at room temperature for up to 30 days.
How long can Mounjaro be kept at room temperature after opening?
Once you inject from a Mounjaro KwikPen for the first time, you can store it at room temperature (below 30°C) for exactly 30 days. After 30 days, dispose of the pen safely, even if medication remains inside. Do not refrigerate an opened pen.
What should I do if my Mounjaro pen has been left out of the fridge by accident?
If an unopened pen has been at room temperature for a short time (under 2 hours), it's likely still safe — ask your pharmacist. If it's been longer, contact your pharmacist or prescriber before using it. Never assume a compromised pen is still effective; it's better to clarify than risk receiving an ineffective dose.
Ready to take control of your weight loss journey?
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